A common optical recording medium is the well-known compact disc (CD). Over the years CD format has evolved from the initial CDDA audio format (Red Book) to a whole family of formats covering different audio, video, and data applications. Some of these formats support a mixture of CDDA format and a variety of data and video formats on the same disc. Examples for such formats are Mixed-mode CD, CD Extra, and VCD.
Format Definitions:
CDDA:
CDDA format is the so-called compact disc (audio CD) and is specified in the IEC 908 standard (known as Red Book format).
CD-ROM/CD-ROM-XA:
The CD-ROM/CD-ROM-XA format describes the extension of CD to store computer data (known as Yellow Book format).
Mix-Mode:
Mix-mode format comprises a combination of CDDA and CD-ROM/CD-ROM-XA tracks, usually one data track (files) plus up to 98 audio tracks.
AVCD:
The Audio Video CD comprises an ISO9660 track (file system) plus a video track plus CDDA tracks.
CD Extra/CD Plus/Enhanced CD:
These discs contain a first session comprising CDDA and a second session comprising data. The format is defined in the Blue Book.
VCD—Movie:
The Video CD (Movie) comprises an ISO9660 track (file system) plus a video track. The format is defined in the White Book.
VCD—Karaoke:
The Video CD (Karaoke) comprises an ISO9660 track (file system) plus a video track. The format is defined in the White Book.
While copying of audio CDs was only a minor problem for a long time, the recent availability of inexpensive recording equipment and media has made unauthorised copying of audio CDs a major issue for the music industry. Therefore, a number of copy protection mechanisms have been introduced. The copy protection mechanisms shall prevent copying audio CDs using a CD-ROM drive of a personal computer. Copy protected discs will disturb the CD-ROM drive if it tries to copy the digital data format directly. Furthermore, depending on the copy protection mechanism, it will also cause playback problems. However, the copy-protected discs must not cause any side effect or degrade the quality of the audio tracks if the disc is played back with a standard audio CD player. The copy protection mechanism may, therefore, not violate the CD Red Book standard too much and exceed the tolerances of a standard audio CD player.
However, caused by the large number of different disc formats, many multi-format disc players are available on the market. In addition, due to the increasing success of compressed audio formats like MP3, there is a growing demand for disc players capable of playing back compressed audio files. For the above reasons many current disc players are equipped with CD-ROM drives rather than standard audio CD drives as used in older audio CD players. These disc players are confronted with severe problems upon attempting the legitimate playback of copy-protected audio CDs.
Known copy protection mechanisms can be classified into five categories, which are listed below:    1. Hidden audio tracks in the table of content (TOC)
This mechanism uses multiple sessions. A first session contains audio tracks, which are marked as digital tracks in the TOC. The second or later session contains a number of virtual audio tracks. Therefore, the CD-ROM drive cannot identify the audio tracks and cannot find the file system in the first session. It only finds the virtual audio tracks in the second or later session. Standard audio CD players do not have any problems with this mechanism, since they only support a single session and will ignore the indication as digital tracks. A standard audio CD player will persist to process any track it encounters as an audio track.    2. Wrong or incomplete file map
A first session contains the audio tracks. A data session is located in a second or later session and pretends an existing file system. The CD-ROM drive is trapped in this second or later session upon trying to parse a wrong or incomplete file map. This mechanism does not disturb a standard audio CD player either, since the first session is correct and the player will ignore any further session.    3. Noise insertion by emulation of missing frames
The absolute and/or relative time in the Q sub-channel is modified in regular intervals to emulate missing frames. The CD-ROM drive will try to retrieve the missing frames to assure the integrity of the output data stream. As a result, the drive will either abort this operation due to excessive reading errors (C2 error) or the data output will be interrupted or chopped up into discontinuous pieces. A standard audio CD player will simply ignore the missing frames.    4. Unrecoverable errors in the audio track
Unrecoverable errors are inserted in the audio track by scrambling the error correction code (ECC), by adding defects during the 8-to-14 modulation (EFM), or by inserting artificial fake errors in the audio content (C2 error). This technique decreases the readability of the disc and is based on the fact that CD-ROM drives use a more sophisticated error correction approach than audio CD drives. A standard audio CD player will rather try to hide the errors instead of making too many correction attempts.    5. Unsettling the CD writer
Using inconsistent pre-gap lengths and leaving index marks in unexpected positions will unsettle many CD writers. Moreover, modifying the TOC in a second or later session to indicate a full data size larger than the available disc capacity, for example 900 MB, will further unsettle many CD writers and keep them from making a digital copy.
The following table shows a summary of existing copy protection methods with the regions of a disc, to which they can be applied. Furthermore, the type of error caused by the copy protection methods and the potential impact on the sound quality is also shown.
In the table, the protection types in the last column and their effect on playback by a CD-ROM drive can be explained as follows.
Track copy protection (usually mechanism 4): Each track has to be protected individually. The tracks can be played back by any CD-ROM drive (in CDDA mode) since the TOC in the lead-in area allows to precisely determine the disc type as CDDA.
Quality copy protection (usually mechanism 3): The tracks can be copied but their quality is decreased. The sound will comprise random pauses or any other interruptions, or will be discontinuous. Similar to the track copy protection, the tracks can be played back by any CD-ROM drive (in CDDA mode) since the disc type can be determined as CDDA.
Disc copy protection (usually mechanisms 1, 2, 5): The whole disc is protected against copying. However, it cannot be played back in a CD-ROM drive since the TOC does not allow to determine the disc type as CDDA.
1st sessionLater session(s)RemarksRegions inLead-Lead-Lead-ProgramLead-PotentialdiscinAudiooutinArea/outsound qualityProtectionMechanismAreaTrackAreaAreaAudio TrackAreaEffectimpactType1st sessionMix content TOCXHide audioNoDiscWrong Index numberXUnsettleNoTrackExtra frame(noise)XC2 errorNoTrackEFM errorXXC2 errorYesTrackScramble ECCXXC2 errorYesTrackPre-gap lengthXUnsettleNoQualityShuffle audio and dataXXXHide audioNoQualityAbsolute timeXXUnsettleNoQualityRelative timeXXXUnsettleNoQualityLater session(s)Incorrect TOCXUnsettleNoDiscIncorrect file mapXUnsettleNoDiscExtra frame(noise)XC2 errorNoDiscEFM errorXXXC2 errorNoDiscScramble ECCXXC2 errorNoDiscPre-gap lengthXUnsettleNoDiscShuffle audio and dataXXXHide audioNoDiscVirtual audio/dataXUnsettleNoDiscOver 99 audio tracksXUnsettleNoDiscPoint 1st session trackXUnsettleNoDiscShorter than actualXUnsettleNoDiscAbsolute timeXXXUnsettleNoDiscRelative timeXXXUnsettleNoDisc
Predictably, the copy protection mechanisms are not limited to the existing known approaches. A large number of further mechanisms may be developed. However, the copy protection mechanisms must not disturb playback of the audio CDs in standard audio CD players. Therefore, they cannot violate the red book standard too much and exceed the tolerances of a standard audio CD player. For this reason the copy protection mechanisms only make a few modifications to the lead-in area of the first session, as can be seen from the above table. In addition, the mechanisms must also meet market requirements. Consumers will not accept a degradation of the sound quality or a data track before the audio tracks in a single session CD, which could be used to trap a CD drive, but which would be played back as a short mute track by a standard audio CD player. In an alternative approach, a copy protection mechanism could be based on combining a plurality of audio tracks in a single track, and using indices instead of tracks for random access. However, many audio CD players, especially low-end players, do not support index play.
As described in the list of copy protection mechanisms, most problems do not occur with audio drives. It would, therefore, be desirable to operate a CD-ROM drive in an audio drive mode. For this purpose, however, the format of an optical disc inserted in the CD-ROM drive has to be securely determined.
JP 10 124 978 discloses a method and an apparatus, which allow to discriminate whether a specific track of a CD-ROM is a data track or in fact an audio track conforming to the CDDA format. This discrimination enables a multi-format disc player to reproduce audio tracks stored on the CD-ROM while at the same time preventing the erroneous reproduction of data tracks stored on the CD-ROM. The discrimination between both types of tracks is based on additional four-bit control attributes for each track, which are included in the table of contents (TOC) of the disc, indicating if a track is a data track or an audio track. This additional attributes are evaluated by an information class distinction means before starting reproduction of the tracks, and only audio tracks are reproduced. The disclosed method has the disadvantage that if the information in the TOC is partially incorrect due to copy protection measures, discrimination between audio tracks and data tracks might fail and some audio tracks might not be reproduced.
It is, therefore, an object of the present invention to propose a method for determining the format of an optical recording medium comprising one or more data, audio and/or video tracks in one or more sessions, which is robust against partially incorrect data on the recording medium. It is another object of the present invention to propose an apparatus for reading from and/or writing to optical recording media using such method.